This is a revised application to renewal a grant focused on understanding the genetic basis of cholestasis, i.e. impairment in bile formation and/or bile flow. The current award was used to isolate a gene which, when mutated is responsible for two forms of inherited cholestasis, benign recurrent intrahepatic cholestasis (BRIC) and progressive familial intrahepatic cholestasis, type 1 (PFIC1). The main goal of the renewal will be to characterize the function of this gene (termed FIC1) by a variety of in vitro and in vivo experiments. A first step will be to identify specific FIC1 mutations responsible for disease in a series of BRIC and PFIC1 patients for whom DNA samples have been collected but in whom mutations have not yet been identified; this mutation analysis will show whether particular FIC1 genotypes are correlated with specific clinical features of BRIC and PFIC1. Screening for FIC1 mutations will also be undertaken in patients with apparently sporadic forms of cholestasis. Mice will be generated in whom the murine ortholog of FIC1 (Fic1) has been disrupted. These mice will be used for anatomic, physiologic, and pharmacological studies aimed at understanding the normal function of FIC1 and as a model for studying cholestasis. These studies will complement efforts by collaborators who will use other approaches to identify the function of FIC1. It was shown recently that mutations in genes other than FIC1 cause forms of cholestasis that are distinct cl8inically, histopathologically, and biochemically from BRIC and PFIC1. Characterizing the function of these genes in conjunction with that of FIC1 will illuminate the molecular basis of normal bile formation and bile flow.